In complex electronic systems of today intelligent power management is crucial for the function of the system. More and more switched mode power supplies are used with different supply voltages. In order to provide 100% uptime of the electronic system hot-swapping of circuits are necessary. If a circuit with high in-capacitance is hot-swapped a large inrush current might occur, if the capacitors of the circuit are discharged. This large inrush current might cause fluctuations of the power supply voltages. This phenomenon is commonly designated ‘sags’.
Another problem involves power line disturbances (PLD) that can involve large voltage spikes on the supply means caused by for example inductive load switching. This phenomenon is commonly designated ‘surges’.
The same equipment that causes sags when turned on can often cause surges when turned off.
These disturbances can have a detrimental effect on computers, memory circuits and radio communication equipment as well as other types of electronic circuits. It is therefore a large need to mitigate the effects due to these disturbances.
There exists a vast array of methods for mitigating the effects of PLD and hot-swapping. Among the most popular methods are the linear mode of control and the switched mode of control of over currents.
The linear mode of control is used in several integrated hot-swap power managers, such as the TPS2394 from Texas Instrument. This circuit utilizes a power FET to provide load current slew rate control and peak current limiting.
However, the linear mode of control involves high power dissipation during a short time interval in the power FET, if the disturbance is large. Hence, large and expensive power FET's are needed in order to stay within stated safe operating area (SOA).
The other popular method is the switch mode of control; this method is disclosed in for example U.S. Pat. No. 6,841,979. This disclosure describes the use of a pulse width modulator to control the current, thus decreasing transistor power losses.
A disadvantage associated with the use of switch mode control of the current is the large amount of conducted or radiated noise that is generated by the switching.
Therefore, a large need for an improved current control circuit that obviates the need for large power FET and reduces the amount of generated noise exists.